Generally, a notebook computer utilizes LCD(Liquid Crystal Display) as a display device. The LCD does not provide light source by itself. Light must be provided by a backlight module which contains a special gas which radiates light when actuated by high voltage of high frequency which is generated by means of a backlight converter, such as DC/AC converter, in order to allow the LCD emit light to display message. For the notebook computer, the power may be supplied either by an AC/DC adapter or an electrical cell. In case of AC/DC adapter, AC voltage is converted into DC voltage without any time limitation. However, if the cell is used, its use is limited in view of using life because the charging capacity of a cell is limited so that the cell cannot provide a long-term power to the notebook computer. To solve the above problem, most of the computer manufacturers intend to use cells having longer life, but they did not found out the other main factor which affects the life of cell and which cannot extend the using life of cell, i.e. the efficiency of converter is low.
A problem is encountered upon designing high efficiency converters. If the problem cannot be solved, the desired aim of excellent efficiency and low cost cannot be achieved. The problem is that the working range of input power voltage of the converter cannot be too braod. The efficiency of the converter is determined by the equation .vertline.Vmax/Vmin.vertline.&lt;=N (some manufacturers may set N to be 2). From the forgoings, it is known that for the notebook computer, two kinds of power supply modes, including AC/DC adapter and cell, may be used. Because the AC/DC adapter is the charging device for providing power to rechargeable cell, the voltage level of the input of AC/DC adapter should be higher than the maximum voltage level of the rechargeable cell, i.e. Vmax. Vmin is the minimum level at which the cell can discharge. Under this condition, .vertline.Vmax/Vmin.vertline.&gt;=2, and the ideal ratio N cannot be obtained. If the ratio N is to be lowered, an additional circuit should be designed and added to allow the ratio N to approach ideal value, and therefore the cost of the adapter is increased. The notebook manufacturers still endeavor to increase the using efficiency of converter and to approach the ideal ratio N at the cost of lowest price.